This invention relates to a process for preparing organohalosilanes according to the Rochow reaction and a copper catalyst used therein.
In the industry, the Rochow reaction is typically employed for the synthesis of organohalosilanes such as methylchlorosilanes. That is, direct reaction of organic halides such as alkyl halides and phenyl halides with metallic silicon particles is carried out at 250 to 500xc2x0 C. in the presence of a copper catalyst. The key technology in this reaction for the synthesis of methylchlorosilanes is to increase the selectivity of the most demanded dimethyldichlorosilane. And for the synthesis of phenylsilanes, the key is to yield desirable diphenyldichlorosilane and phenyltrichlorosilane in a composition complying with the demand.
More particularly, organohalosilanes are synthesized by the Rochow reaction of passing an organic halide gas such as methyl chloride through a contact mass comprising metallic silicon, copper catalyst and a minor amount of co-catalyst to effect direct reaction in a gas phase. In this reaction, it is important to increase the reaction rate of metallic silicon because the cost of metallic silicon is predominant among the raw material cost. Since a variety of by-products usually form in addition to the desired diorganodichlorosilane, it is also important to control reaction conditions so that the proportion of these by-products may comply with the supply/demand balance of organochlorosilanes. Industrially, this reaction is generally carried out in a reactor such as a fluidized bed, vibrating fluidized bed or agitating fluidized bed while replenishing the contact mass to the reaction system. Requisite considerations on this reaction are to reduce the time taken for activation until the reaction reaches a steady state (that is, activation time), to prevent lowering of catalytic activity due to deposition of deactivated contact mass with the progress of reaction (that is, lowering of reaction rate and selectivity), and to minimize the amount of reactor residues (high-boiling fractions such as disilanes) which are unnecessary.
However, the conventional Rochow reaction requires a very long time for activation until the reaction reaches a steady state. The steady state, in turn, is relatively short. The yield of dimethyldichlorosilane decreases with the lapse of time because of carbon deposition on the contact mass surface due to side-reaction. In the synthesis of methylsilanes, for example, high-boiling fractions such as disilanes and undesired products such as methyltrichlorosilane increase due to side-reaction, resulting in low reaction yields.
JP-A 10-309465 discloses that it is effective to add a copper catalyst to this reaction system after metallic silicon power alone is previously heated near the reaction temperature. As understood from this disclosure, only the active copper catalyst is necessary. The deactivated catalyst merely contributes to the formation of carbon and side reaction to increase the amount of Me(H)SiCl2 and the ratio of MeSiCl3/Me2SiCl2. It is then important to reduce the residence time of the copper catalyst and remove the copper catalyst once deactivated.
However, merely reducing the particle size of the copper catalyst is ineffective for reaction in a fluidized bed or agitating fluidized bed because the catalyst is carried with the stream of unreacted reactant gas and produced silane vapor away from the reaction system without acting as the catalyst. It is thus desired to have a catalyst which resides within the system while it is active, but once deactivated, is immediately discharged out of the system.
In the reaction of this sort, the copper catalyst is mixed with metallic silicon powder and acts on an alkyl halide (e.g., methyl chloride) or aryl halide (e.g., benzene chloride) to produce a corresponding organohalosilane. This reaction is basically a gas-solid heterogeneous reaction between the organic halide which is gaseous and the copper catalyst and metallic silicon powder which are solid even at elevated temperature. It is then reasonably anticipated that the surface activity of the copper catalyst is important. With respect to the use of copper oxide as the catalyst in this reaction, powder parameters of the catalyst including surface area and particle size are referred to in U.S. Pat. Nos. 4,520,130, 4,504,597 and JP-A 9-173844. However, the morphology of the copper catalyst has not been discussed from the above-mentioned standpoint, much less the morphology of metallic copper catalyst.
An object of the invention is to provide a novel and improved metallic copper catalyst for use in the synthesis of organohalosilanes, which can be effectively discharged out of the reaction system once deactivated. Another object is to provide a process for preparing organohalosilanes using this catalyst.
With respect to the copper catalyst for use in the Rochow reaction, a study was made on the morphology of the copper catalyst which can be readily discharged out of the reaction system when it is deactivated so that it becomes useless for the Rochow reaction and rather causes side reaction. It has been found that when a copper powder in flake (metal foil) or scale form having a large surface area and a low bulk specific gravity which is available as ground or chopped powder of rolled metallic copper foil, or stamped copper powder obtained by stretching and milling rolled copper foil or machined copper plates as by stamping is used as the copper catalyst, this catalyst can be quickly discharged out of the reaction system once deactivated.
In a first aspect, the invention provides a metallic copper catalyst for use in the synthesis of organohalosilanes, consisting of a metallic copper powder in flake or scale form having a bulk specific gravity of 1 to 3 g/cm3 and a mean particle size of 10 xcexcm to 1 mm as measured by laser diffraction particle size distribution analysis. Preferably, the metallic copper powder is a copper foil powder or stamped copper powder having a specific surface area of 0.1 to 2 m2/g as measured by the BET method or air permeability method.
In a second aspect, the invention provides a process for preparing organohalosilanes, comprising the step of reacting an organic halide with metallic silicon particles in the presence of the metallic copper catalyst defined above.
In connection with the Rochow reaction, that is, the organohalosilane synthesis reaction between an alkyl halide (e.g., methyl chloride) or aryl halide (e.g., benzene chloride) and metallic silicon in the presence of a copper catalyst and a co-catalyst (e.g., zinc or tin), the present invention uses the above-defined catalyst to solve the problem of the prior art technology that the activation time (or induction period) taken until the reaction rate and selectivity of silane synthesis reach a steady state is very long while the steady state continues relatively short. By studying the catalytic action and physical properties of the copper catalyst, the invention has succeeded in optimizing the morphology of a metallic copper catalyst. The invention overcomes the above-mentioned problems of the Rochow reaction and in particular, increases the selectivity of desired dimethyldihalosilane, and prolongs the steady state or increases the conversion rate of silicon, achieving improved reaction yields.
More particularly, the feature of the invention resides in the removal of the deactivated contact mass, especially deactivated copper catalyst from the reaction system. For the reaction to which the invention pertains, the copper catalyst is an essential component which is mixed with metallic silicon powder and helps the silicon react with an alkyl halide (e.g., methyl chloride) or aryl halide (e.g., benzene chloride) to form corresponding organohalosilanes. Since this reaction is basically a gas-solid heterogeneous reaction between the organic halide which is gaseous at elevated temperature and the copper catalyst and metallic silicon powder which remain solid even at elevated temperature, the copper catalyst must present a fresh active surface. However, the copper catalyst surface is deactivated with the progress of reaction. If the copper catalyst is merely deactivated from the reaction, it will give rise to no problem even when it continues to reside within the reaction system. On analysis of reaction states including produced silane composition and carbon deposition on the contact mass, however, it was found that in fact, the deactivated catalyst maintains activity to side reaction. It is, therefore, crucial to remove the copper catalyst from the reaction system immediately after deactivation.
The problem can be solved using copper powder (catalyst) in microparticulate form. However, simply using microparticulate copper powder is uneconomical because the majority of the copper powder is carried along with the stream of reactant gases and reaction products away from the reaction system before it exerts catalysis, which necessitates to feed a great excess of the copper catalyst. On the other hand, if copper particles having a large particle size are used, they will deposit and accumulate in the reaction system, giving rise to the above-mentioned problem. The particle size of copper powder that can clear both the problems is limited to a narrow range. In the Rochow reaction in which the particle size of metallic silicon reactant changes with the progress of reaction, it is in fact difficult to control the particle size of copper powder. This difficulty is eliminated by using as the copper catalyst a metallic copper powder in flake or scale form having a bulk specific gravity of 1 to 3 g/cm3 and a mean particle size of 10 xcexcm to 1 mm as measured by laser diffraction particle size distribution analysis, and more preferably, copper foil powder obtained by milling electrolytic copper foil or rolled copper foil, or stamped copper powder obtained by stretching and milling electrolytic copper foil, rolled copper foil or machined copper powder as by stamping, in flattened scale or flake form having a large specific surface area of 0.1 to 2 m2/g as measured by the BET method or air permeability method. That is, the copper catalyst in a very thin scale or flake form is preferable as the catalyst for the direct silane producing method or Rochow reaction. This copper catalyst has a large air-permeability method specific surface area and/or BET method specific surface area, and once deactivated by reaction in the fluidized bed or agitating fluidized bed, it converts into fine particulate form so that it may be carried along with the stream of produced halosilanes and unreacted reactant gases and vapors away from the reaction system.